Ultrasonic transducers are used in many medical applications and, in particular, for the non-invasive acquisition of images of organs and conditions within a patient, typical examples being the ultrasound imaging of fetuses and the heart. The ultrasonic transducers used in such applications are generally hand held, and must meet stringent dimensional constraints in order to acquire the desired images. For example, it is frequently necessary that the transducer be able to obtain high resolution images of significant portions of a patient's chest cavity through the gap between two ribs when used for cardiac diagnostic purposes, thereby severely limiting the physical dimensions of the transducer.
As a consequence, and because of the relatively small aperture between human ribs and similar constraints upon transducer positioning when attempting to gain images of other parts of the human body, there has been significant development of linear or phased array transducers comprising multiple transmitting and receiving elements, with the associated electronics and switching circuits, to provide relatively narrowly focused and "steerable" transmitting and receiving "beams". The most common of such transducers comprises a one element wide by multiple element long linear array of transmitting and receiving elements, thereby allowing the beam to be focused and scanned along the plane of the transducer elements, which is referred to as azimuth scanning. The elements of such transducers are often arranged in line along a flat plane or may be arranged along concave or convex contours, thereby providing a greater scanning arc, and are often provided with acoustic focusing lenses made of materials having suitable properties to act as lenses in the acoustic frequency ranges of interest.
The transducer elements of linear or phased array transducers are most frequently made of a piezoelectric material and the linear array of elements is generally mounted onto a body made of some backing material. One or more layers of impedance matching material, generally considered to be a part of the elements themselves, is often superimposed upon the transducer elements. A lens comprised of a suitable material may be additionally superimposed upon the impedance matching material to shape or focus the beams generated by the transducer elements, or the impedance matching material may have suitable characteristics and may be shaped to operate as an acoustic lens. Connections between the individual transducer elements and the associated electronics and switching elements are usually provided through various arrangements and combinations of thick and thin film circuits, flexible printed circuits and wires.
Such transducers are generally constructed from a single piece of transducer material having a width equal to the length of one element and a length equal to the widths of the total number of elements plus spaces between the elements. One or more thin or thick film circuits or flexible printed circuits having connections and paths for the individual elements, or the like implemented in any of several other ways, are bonded to one side of the piece of transducer material and a layer or layers of matching material may be bonded to the stack as required by the final construction of the transducer. A temporary or permanent layer of backing material of some form, such as a flexible material, may also be bonded to the back of the stack to aid in handling the stack during manufacture.
Successive cuts are then made across the width of the transducer stack at intervals corresponding to the widths of the elements and the spacing between the elements to divide the single piece of material into the individual elements. This operation is generally referred to as "dicing" and is usually done with a device referred to as a dicing saw, but may be done with other techniques, such as lasers. These cuts may extend only through the transducer and matching material layers, or partly or completely through the circuit layer, or through the circuit layer and at least a part of any backing layers, depending upon the detailed design and implementation of the circuit layers. The assembly of individual transducer elements with the circuit and matching layers are then bonded to the backing body, which may have a flat, concave or convex face, as described above, with any temporary backing layers being removed as necessary. It should be noted that in certain instances the dicing may be done after the assembly of transducer elements, matching materials, and circuits is bonded to the backing material and that the dicing cuts may extend into the layers of backing material or even into the backing body.
The connections to wires or printed circuits, such as flexible circuits, which in turn connect to the electronics and switching elements are made before or after the transducer assembly is bonded to the backing body, again depending on the detailed design and implementation of these connections.
While such azimuth scanning linear or phased arrays are advantageous for the intended purposes of such ultrasonic transducers, this type of linear or phased array transducer has the disadvantage that it can perform only an azimuth scan along the single plane of the transducer elements and is thereby often referred to as a one dimensional, or "1D", array. As a consequence, there have been many attempts to provide linear or phased array transducers that are also capable of scanning or focusing in elevation as well as azimuth, that is, along the axis at right angles to the azimuth plane along which the elements are arrayed, as well as along the azimuth plane.
As is well understood, the formation, steering and focusing of the transmitting and receiving beams of a transducer are controlled by selection and use of the various separate physical divisions or areas of transducer material comprising the transducer array, which are referred to as apertures. In a "1D" array, which is capable of forming beams only along the azimuth scan plane, each transducer element forms a single aperture, so that the array is formed of a single row of apertures extending along the face of the array, and such transducers are referred to as "single aperture" transducers.
In contrast with a "single aperture" transducer, in which each aperture is formed by an element extending across the face of the array as a single unitary area or division, each corresponding element in a transducer capable of scanning in elevation is divided into multiple sub-areas, or elevation sub-segments, and thereby into multiple apertures. Such transducers are consequently referred to as "multiple aperture" transducers, or two dimensional or "2D" transducers. The shape, focus and direction of the scan planes and beams of a multiple aperture transducer are again controlled by selecting combinations of the apertures, that is, of the sub-areas of the transducer elements, so that the apertures of a multiple aperture array can be used to steer and focus the transducer scan beam along the elevation axis as well as along the azimuth axis and can define multiple azimuthal scan planes, each being at a different angle of elevation. The apertures may be either driven actively, or simply de-activated to reduce the size of the acoustic aperture, thereby controlling the shape, direction and focus of the transmitting and receiving beams formed by the transducer array.
While the construction of the piezoelectric elements for a multiple aperture transducer presents greater difficulties than for a single aperture array, the methods are well known and similar to those used in construction of a single aperture array. For example, a particular application may require that each element be comprised of three segments, or apertures, that is, two outer segments and a middle segment. This may be achieved, for example, by constructing the transducer elements from three elongated pieces of transducer material, that is, two outer pieces and a middle piece, and then dicing the pieces across the face of the array as was described with regard to single aperture arrays, or by additional cuts along the transducer stack in the longitudinal direction to divide the two outer segments from the middle segment.
The primary problem in constructing multiple aperture arrays is that the number of electrical connections to each element, each of may be comprised of three or more segments, and possibly between elements, is greatly increased while the space in which to make the connections does not increase. For example, going from a single aperture array to a three aperture array triples the number of segments in each elements, that is, from one to three, and, instead of one connection to the single segment comprising the element, there must be two separate connections to the two outer segments, a third connection to the middle segment, and additional connections to each possible pair of segments. In addition, each middle segment is bounded on both ends by the outer segments of the element and on either side by the two adjacent elements, so that the middle segments are not readily accessible for connections. In addition, the connections to the outer and middle segments must be made in such a manner as not to interfere with the acoustic characteristics of the transducer.
Considering a specific example, the Hewlett-Packard Model 21215 transducer provides two sizes of elevation apertures and is constructed generally as described above, that is, of a linear array of separate or separated elements wherein each element is comprised of three separate segments, two outer segments and a middle segment. In this design, the elements are arranged in a straight plane, rather than a concave or convex arc, and the middle segment of each element is connected to a transmit/receive circuit while the two outer segments of the element are connected together and then to a second transmit/receive circuit or through a switch to the same transmit/receive circuit as the middle segment.
Connections to the segments are made through flex circuits, that is, circuits etched onto thin, flexible circuit boards, and an individual flex circuit is used for each set of elevation segment connections wherein each flex circuit contains all of the connections for the corresponding segments of each of the elements along the array. The transducer therefore requires three flex circuits, one for each out row of segments and one for the middle row of segments. The two flex circuits connecting to the outer segments of each element of the outer segments and are then connected by a jumper flex circuit or a circuit board. The third flex circuit connects to the middle segments of the elements, and thus must make connection at the middle of the back side of the piezoelectric array.
It is therefore apparent that a two aperture array requires three times as many connections to the piezoelectric segments themselves, twice as many flex circuits as in a single aperture array, and two additional flex circuit to flex circuit connections by a flex jumper or printed circuit board for each element. These connections result in higher cost and lower manufacturing yield and reliability. In addition, assembly is more complex in that the flex circuit to the middle segments must be carefully aligned with the flex circuits to the outer segments. This factor alone makes it difficult, if not impossible, to manufacture a curved array and the presence of the middle segment flex circuit requires the user of either a poured backing body material or complex molding or machining to manufacture the backing body.
The methods used in the prior art to construct multiple aperture arrays include the use of multiple flex circuits, as described just above, connections embedded in the backing body, and the use of electrostrictive rather than piezoelectric materials for the transducer elements.
The disadvantages of multiple flex circuits have been discussed above, and the disadvantages of connections embedded in the backing body are comparable. For example, a preferred method for embedding connection circuits in the backing body is the use of laminating layers of backing material on or in which signal leads have been made. An alternative is the use of a multi-layer thick film ceramic hybrid circuit which also serves as the backing body. The laminated layers with embedded connection circuits results in leads which run vertically, that is, perpendicularly, between the segments and an interface circuit to which the connections are made, but also results in leads with very small cross sections that are attached at both ends by butt joints, which lack reliability. The use of a multi-layer thick film circuit, in turn, can provide much stronger and more reliable connections, but the acoustic characteristics of the ceramic material degrades the acoustic performance of the transducer. Both approaches, moreover, have the disadvantage of requiring multiple steps to make the connections to the piezoelectric elements and the added cost from not using standard printed or hybrid circuit manufacturing techniques.
Certain other transducer designs have used electrostrictive materials rather than piezoelectric materials for the elements because electrostrictive materials require only one signal lead for each element as the aperture can be controlled by switching entire segments of the elements on or off with a dc bias voltage.
The present invention provides a solution to these and other problems of the prior art.